Friendlier migrations happy customers

[callum-mcdata]

Addresses #5807

Description

Following the helpful direction in @jtcohen6's gist, this DRAFT PR WHICH SHOULD NOT BE MERGED OR EVEN JUDGED, maps the old attributes in the manifest to the new ones and overwrites the expression metric type to derived.

What it is currently missing is some form of warn message at compile. I will need to add that before I can start addressing any weaknesses it may be missing in the tests.

I've tested this in a local branch and on a test dbt project to confirm that the manifest produced does look correct even with the old metric spec.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[github-actions]

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[jtcohen6]

@callum-mcdata I completely missed seeing this... check out #5825!

[jtcohen6]

Closed in favor of #5825